One of the worst tragedies to happen to writing is that the phrase “flat character” has become an insult.
“But it is an insult, isn’t it?” I can hear you saying. “Doesn’t everyone want their characters to be fully realized, three-dimensional people who stay with the reader long after the book has been put down?”
Well, yes. But not all of your characters. And, at any rate, that’s not what being a flat character is about.
What differentiates a three-dimensional character from a flat one is the ability to grow and change. To use an example from classic literature, in Romeo and Juliet, Juliet is a three-dimensional character. She starts as a shy, young girl, who hasn’t thought much about what she wants from the future and just wants to please her parents. By the second act, she’s fallen in love with Romeo and is conspiring with him and the nurse to arrange a secret marriage. And then, when Romeo is exiled and her parents are intending to force her into a bigamous marriage with Paris, she defies both them and the nurse to fake her own death, having decided that her loyalty to Romeo overrides everything else in her life.
Note that nothing I’ve said above should necessarily be taken as an endorsement of Juliet or a reason to like her. It’s simply a fact that, whether you love her or hate her, whether you agree with the choices that she made or not, Juliet is not the same person in Act 5 that she was in Act 1.
An example of a flat character would be the nurse. She’s bawdy, earthy, interested in seeing Juliet married above all else. In Act 1, that means encouraging Lady Capulet’s scheme of a match between Juliet and Paris. In Acts 2 and 3, it means assisting with the marriage scheme, then smuggling Romeo into Juliet’s room so that they can consummate the marriage. And then, after Romeo is exiled, it means going back to encouraging the marriage with Paris. But with all of these different positions she takes and decisions she makes, the underlying character is the same. The nurse is the same person at the end of the play as she was in the beginning.
But, just as I said above that Juliet being a three dimensional character wasn’t a reason to like her, the nurse being a flat character isn’t a reason to dislike her or to think that she’s badly written. Would the play have been better if it had included a fourth act soliloquy from the nurse musing about marital fidelity and whether it would be worth it for Juliet to give up everything to stay with Romeo? Questioning her loyalty to the Capulet family in light of Lord Capulet’s threats against Juliet? I don’t think so. Romeo and Juliet is the story of, well, Romeo and Juliet. It’s supposed to be about their relationship and growth. Trying to shoehorn in a subplot about how all of this affects the nurse would just get in the way.
That’s a problem I feel that a lot of modern writers have. They don’t want their characters to be flat. But showing character growth takes time and pages. If you insist on making everyone from the innkeeper in the town where the hero grew up to the ferryman who takes him across the river into “well-rounded characters,” the net result is that either the book is so crowded that there’s no room for any character to stand out—or that what should have been a relatively simple story balloons into an epic dozens of books long with no end in sight.
Now, there’s nothing wrong with falling in love with a secondary character and wanting to give him or her more depth. But you can only do this with so many characters. For most of them, what you want is a bit of shading: a line or two that hints that there may be more to this person than the stereotype, and if they were the main character, you’d see something interesting. But you have to keep in mind that this isn’t the main character, and hints are all you should give. Don’t let them take over or occupy page space that needs to be reserved for your actual main characters.
The tl;dr version of all this is that you shouldn’t be afraid of flat characters. It’s okay to have a character who just fills a role in the narrative and never exhibits any growth. A bit of shading on the secondary characters can make your world feel more real and lived-in, but too many “well-rounded” characters will distract from your story and cause unneeded bloat in your book.